Surface Finish Chart —
Ra, RMS, and Rz Conversion with Process Ranges
Ra, RMS, and Rz: What Each Means
Ra (roughness average) is the arithmetic average deviation of the surface profile from its mean line, and it is the default roughness parameter on American drawings. A bare number in a surface finish symbol on a U.S. drawing means Ra in microinches; a bare number on a metric drawing means Ra in micrometres. The two differ by a factor of 39.37, which is the single most common surface finish misread: 32 µin and 0.8 µm are the same finish, but 32 µm would be a very rough one.[1]
RMS (root mean square) is an older parameter that weights peaks and valleys more heavily. For a typical machined surface RMS reads about 11% higher than Ra, so 64 RMS and 58 Ra describe roughly the same surface. RMS callouts persist on legacy drawings; treat an unlabeled callout on an old print with care and confirm which parameter was meant.[1][2]
Rz (mean peak-to-valley height) is common on European and Asian drawings. It measures the height between the highest peaks and deepest valleys over sampling lengths, so it runs much larger than Ra for the same surface: typically 4 to 7 times Ra, depending on the process. There is no exact conversion, which is why our converter reports Rz as a range. If a drawing specifies Rz, measure Rz.[1][2]
Surface Finish by Machining Process
Each process has a natural finish range. The low end takes sharp tooling, light finishing passes, and rigid setups; the high end is a roughing pass. When a drawing calls for a finish below a process's range, the part needs a second operation, and that is a cost driver worth designing around: 32 µin from the mill is routine, 16 µin usually means grinding, and single digits mean honing or lapping.[1][3]
Surface Finish Conversion Table
The ISO N grades with their Ra equivalents in both unit systems, the approximate RMS value, and the processes that typically produce each finish.[1][2][3]
| ISO grade | Ra (µm) | Ra (µin) | RMS (µin, approx.) | Typical production process |
|---|---|---|---|---|
| N12 | 50 | 2000 | 2200 | Flame cutting, rough sand casting |
| N11 | 25 | 1000 | 1100 | Sawing, torch-cut edges |
| N10 | 12.5 | 500 | 555 | Rough turning and milling, sand casting |
| N9 | 6.3 | 250 | 278 | General roughing passes, drilling |
| N8 | 3.2 | 125 | 139 | Standard as-machined finish: turning, milling, drilling |
| N7 | 1.6 | 63 | 70 | Finish turning and milling with care, reaming |
| N6 | 0.8 | 32 | 36 | Fine machining, reaming, grinding |
| N5 | 0.4 | 16 | 18 | Grinding, fine honing |
| N4 | 0.2 | 8 | 9 | Fine grinding, honing, lapping |
| N3 | 0.1 | 4 | 4.4 | Honing, lapping, polishing |
| N2 | 0.05 | 2 | 2.2 | Lapping, superfinishing |
| N1 | 0.025 | 1 | 1.1 | Superfinishing, optical lapping |
How to Call It Out (and What It Costs)
The finish symbol on a drawing applies to the surface it touches; a symbol in the title block sets the default for everything else. 125 µin Ra is what ordinary machining produces without special effort, and 63 µin Ra is the standard callout for surfaces that matter: sealing faces, bearing seats, locating features. 32 µin Ra is achievable in the machining operation with sharp tooling and finish passes. Below that, plan on grinding or honing as a separate operation with its own setup and cost.[3]
The practical rule is the same one that governs tolerances: specify the roughest finish that works. A 16 µin callout on a clamped, painted bracket face adds grinding time and inspection burden for a surface nobody will ever measure. And remember that machining finish and applied coatings interact: anodize slightly roughens a surface, plating follows the substrate finish, and a bead blast resets the whole conversation.
Finish verified on real parts
C&W measures surface finish with calibrated profilometers and documents it with each lot under our AS9100D quality system.